In the field of manufacturing technology of electrical and/or electronic circuits, electrically conductive pastes are used, for example, for mounting electronic parts on a wiring substrate.
Such an electrically conductive paste is a pasty material in which the electrically conductive filler particles are dispersed in resin compositions which serves as a binder, which paste does not show electrical conductivity due to the matter that the resin compositions generally do not show electrical conductivity. However, when the resin is heated to be hardened, the electrically conductive filler particles are caused to approach each other or to contact each other, thereby the paste comes to show electrical conductivity after being hardened.
After the electrically conductive paste has hardened, the hardened resin generally resulted in a joint part. In such hardened resin in the joint part, the electrically conductive filler particles are present in a state that the filler particles contact with each other, thereby the filler particles form a connected state in the form that the filler particles contact with each other. Thus, an electrically conductive path is formed in the joint part. For example, when each electrode of a circuit board is electrically connected with each electrode of the electronic parts each other using the electrically conductive paste, the above electrically conductive path is used. Thus, it is preferable to improve the electrical conductivity in the joint part. Therefore, it is preferable to form an electrically conductive path, which has an electrical resistance as low as possible in the electrically conductive paste after hardened. It is considered that such a low electrical resistance of the electrically conductive path can be obtained, which path is formed in the electrically conductive paste after hardened, by increasing the content of the filler or increasing the contact areas/contact probability of fillers in the paste.
However, in the electrically conductive path which is formed by causing each surface of the electrically conductive filler particles to contact each other, there is a certain limit as to the attainable reduction in resistance. In addition, there has been a problem in the joint part, which has been jointed by the electrically conductive paste, that it has not so high reliability when it is subjected to a heat cycle reliability test. That is, the connection resistance value after the test increases compared with the connection resistance value before the test.
Thus, some ideas have been proposed to attain reduction in resistance in the electrically conductive path. Such ideas comprise, for example, adopting an alloy having a comparatively low melting point as a part of the ingredient of the electrically conductive filler, subjecting the alloy having a comparatively low melting point to heating treatment thereby causing the alloy to be molten, and forming an electrically conductive path among the filler particles by causing the molten alloy to connect the filler particles each other (Patent Citations 1 and 2).
Further, relating to the electrically conductive paste for filling up via holes of a circuit board, it is proposed that an electrically conductive paste comprising two kinds of low melting point metal particles A and B, each of which has a melting point lower than 230° C.; and metal particles having a low resistance having a melting point above 230° C. as its electrically conductive filler particles (Patent Citation 3).
[Patent Citation 1]
    Japanese Unexamined Patent Publication (Kokai) No. 10-279903[Patent Citation 2]    Japanese Unexamined Patent Publication (Kokai) No. 2005-089559[Patent Citation 3]    Japanese Unexamined Patent Publication (Kokai) No. 2005-071825